Aim. To study the impact of birth characteristics on the risk of atopic dermatitis in a twin population. Methods. In a population-based questionnaire study of 10,809 twins, 3-9 years of age, from the Danish Twin Registry, we identified 907 twin pairs discordant for parent-reported atopic dermatitis. We cross-linked with data from the Danish National Birth Registry and performed cotwin control analysis in order to test the impact of birth characteristics on the risk of atopic dermatitis. Results. Apgar score, OR (per unit) = 1.23 (1.06-1.44), P = 0.008, and female sex, OR = 1.31 (1.06-1.61), P = 0.012, were risk factors for atopic dermatitis in cotwin control analysis, whereas birth anthropometric factors were not significantly related to disease development. Risk estimates in monozygotic and dizygotic twins were not significantly different for the identified risk factors. Conclusions. In this population-based cotwin control study, high Apgar score was a risk factor for atopic dermatitis. This novel finding must be confirmed in subsequent studies.